The invention relates to a planetary gear made of gearwheels, comprising a sun wheel on a first shaft, a planet carrier on a second shaft, an internal geared wheel on the gear housing or on a third shaft, and step planets, each with two planet wheels of different sizes, interconnected in a torsionally secured or fixed manner, of which wheels the smaller planet wheels intermesh with the internal geared wheel and the larger planet wheels with the sun wheel.
Planetary gears with step planets are well known, they are for example used as propeller gearing in the Rolls-Royce aircraft engine xe2x80x9cTynexe2x80x9d. Compared to a single stage planetary gear with simple planets, i.e. wheels with an effective diameter intermeshing with the sun wheel and the internal geared wheel, with a modest increase in the volume of the gearing and somewhat increased design expenditure, the step planet design allows a significant increase in the transmission ratio, with the shaft torque largely increasing in the same ratio. In the known designs, the two wheels of the step planet are axially directly adjacent to each other as well as connected to form an integral unit, i.e. they are usually made in one part.
Rotary bearing of such integral step planets is usually via one or several plain bearings (slide bearings) in the interior of each of these components as well as via a rigid planet carrier axis leading through the step planet. In the case of roller bearings, one bearing each is provided axially in front of and behind the step planet, because usually the radially larger roller bearings cannot be accommodated in the interior of the planet wheels, especially the smaller planet wheels.
If the two gearing characteristics of a step planet are projected in axial direction in one plane and if the tooth forces resulting from the pressure with the sun wheel and the internal geared wheel (1st and 2nd stage) are analysed according to size and direction, then it becomes evident that the two tooth forces point in the same general direction with the acute angle between their vectors corresponding to the sum of the two pressure angles (approx. 40xc2x0 to 50xc2x0). If the two tooth forces are vectorially added, we arrive at a resultant force significantly higher than that of each of the individual forces. From the point of view of construction this means that the step planet design leads to high bearing forces so that the carrying capacity of the step planet bearings becomes a decisive criterion for the power that can be transmitted by the gearing. Additional loading of the bearings results from the speed-related centrifugal forces acting on the step planets. In this context it must also be taken into account that propeller gearing is to be designed for a service life of for example 30,000 h, which of course also applies to the roller bearings installed.
It is thus the object of the invention to modify planetary gears with step planets in such a way that with only a slight increase in design volume and construction expenditure (number of parts etc.) at a specified transmission ratio, significantly higher power can be transmitted at the best possible efficiency.
The above objects have been achieved according to the invention in a planetary gearing arrangement that generally includes the components and features mentioned above in the Field of the Invention section, as well as the further special features of the invention to be described next.
Each planet wheel is constructed as a separate unit which is rotatably held on both sides, i.e. axially in front of and behind its center plane, in the planet carrier, in each case via at least one roller bearing with high stability under load. Thus for each step planet there are four large-dimension bearing positions with high stability under load, ensuring minimal friction losses. Each bearing position may comprise several individual roller bearings axially one behind the other. The two planet wheels of each step planet are coupled to each other by an additional (fourth) shaft in a manner that is torsionally fixed or secured, but allows limited relative radial and axial movement. Thus, torque transmission from the large to the small planet wheel of each step planet is by positive locking via the additional shaft which is practically only subjected to torsional load. This bridges the axial spacing, increased according to the invention, between the coupled planet wheels of various sizes. Thus the gear according to the invention can also be referred to as a planetary gear with coupled stage planets.